1. Field of the Invention
This invention relates to systems for discovering semi-conducting materials, and more particularly, to methods, materials, and devices for making and screening combinatorial libraries to identify thermoelectric materials.
2. Discussion
In its simplest form, a thermoelectric device comprises a thermoelectric materialxe2x80x94usually a semiconductorxe2x80x94sandwiched between a pair of contacts. When an electrical potential is applied between the pair of contacts, heat flows from one contact to the other through the thermoelectric material. This phenomenon, which is called the Peltier effect, occurs whenever direct current flows through a junction between two dissimilar materials. Similarly, when a temperature difference is applied between the pair of contacts, an electrical potential develops which varies continuously from one contact to the other through the thermoelectric material. This latter phenomenon is called the Seebeck effect. Its size depends on the magnitude of the temperature difference, and like the Peltier effect, on the properties of the thermoelectric materials.
Thermoelectric devices exploit the Seebeck effect and the Peltier effect to generate power and to pump heat and they exhibit certain advantages over conventional compressor-based systems. For example, engineers employ thermoelectric devices to cool small volumes, such as portable food and beverage containers, medical devices, and integrated circuits, which would be impractical to cool with bulky conventional refrigeration systems. Furthermore, thermoelectric heat pumps offer greater flexibility than compressor-based refrigeration systems since thermoelectric devices can heat, as well as cool, by simply reversing the direction of electrical current through the device. Moreover, because thermoelectric devices have no moving parts, they generate power quietly and reliably. Despite these advantages, thermoelectric devices are not used for general purpose cooling or for power generation because they are less efficient than compressor-based systems. Indeed, the most efficient thermoelectric power generators currently operate at about 10% Carnot efficiency, whereas conventional compressor-based systems operate at about 30%, depending on the size of the system.
Since efficiency and performance of thermoelectric power generators and heat pumps depend primarily on the properties of the materials used in the device, researchers continue to search for new, better performing thermoelectric materials. But, progress has been slow. Indeed, Bi-Sb-Te alloys remain the most efficient room temperature thermoelectric materials available, though they were first used in thermoelectric devices more than thirty years ago.
The slow pace of discovery is due, in part, to the time and expense of synthesizing and testing thermoelectric materials using conventional techniques. In traditional material science, researchers synthesize a few grams of a candidate material that they test or screen to decide whether it warrants further study. For thermoelectric materials, synthesis involves a labor- and time-intensive alloying process. Since material properties often depend on synthesis conditions, the discovery process usually includes a lengthy search for optimum heating and quenching cycles. In many cases, dopants are added to control microstructure, which further increases complexity of the discovery process. Although in recent years scientists have acquired a better understanding of how material structure and carrier concentration influence thermoelectric variables such as thermoelectric power, thermal conductivity, and electrical resistivity, discovery efforts continue to rely heavily on experiment.
Combinatorial chemistry is one approach for accelerating the discovery of new thermoelectric materials. It is a powerful research strategy when used to discover materials whose properties, as with thermoelectric compositions, depend on many factors. Researchers in the pharmaceutical industry have successfully used such techniques to dramatically increase the speed of drug discovery. Material scientists have employed combinatorial methods to develop novel high temperature superconductors, magnetoresistive materials, phosphors, and catalysts. See, for example, co-pending U.S. patent application xe2x80x9cThe Combinatorial Synthesis of Novel Materials,xe2x80x9d Ser. No. 08/327,513 (a version of which is published as WO 96/11878), and co-pending U.S. patent application xe2x80x9cCombinatorial Synthesis and Analysis of Organometallic Compounds and Catalysts,xe2x80x9d Ser. No. 08/898,715 (published as WO 98/03521), which are both herein incorporated by reference.
The use of combinatorial materials science should enable researchers to undertake an efficient, systematic and comprehensive search of new semi-conducting or new thermoelectric materials without many of the problems associated with traditional materials development.
The present invention generally provides a method for discovering semi-conducting or thermoelectric materials using combinatorial techniques. The method includes preparing a combinatorial library of materials, and identifying library members that are semiconductors. The method selects library members that are semiconductors because, currently, the most efficient room temperature thermoelectric materials are narrow band gap semiconductors. Nonetheless, this invention is generally useful for researching for semi-conducting materials, despite this specification focusing on thermoelectric materials. The combinatorial library is typically prepared by depositing library members on a substrate using physical vapor deposition (PVD) or sol-gel or liquid dispensing techniques. Useful PVD techniques include pulsed laser deposition, magnetron sputtering, thermal evaporation and co-deposition.
Identification of semiconductors includes exposing members to radiation of varying wavelength, and measuring reflectance, or reflectance and transmittance, of incident radiation. The radiation may be from the infrared, visible or ultraviolet ranges, depending on the band gap for the semi-conductor that is being researched. For thermoelectric materials, infrared radiation is useful for reflectance and transmittance measurements, with the method determining an optical band gap of each of the library members: the method selects as semiconductors library members having band gaps in the range of about 0.05 eV to about 0.9 eV (30 microns to about 1 micron). Alternatively, identification may include determining, from reflectance and transmittance measurements, ratios of charge carrier density to quasiparticle effective mass of the library members. If transmittance measurements are not available, e.g., as in the case of an IR opaque substrate, the method uses reflectance measurements alone to identify library members that are semiconductors. For example, the method includes selecting members of the combinatorial library of materials exhibiting reflectance versus incident IR energy (wavelength) curves that are characteristic of a semiconductor. Such curves generally exhibit a local minimum in reflectance.
Alternatively, or in addition to identifying semiconductors, the method may include determining a thermoelectric figure of merit, ZT, for each member of a combinatorial library of materials. To determine ZT, the method includes applying an oscillatory voltage, having a reference frequency xcfx890, across each library member, measuring power dissipated by the library members while the oscillatory voltage is applied, and calculating ZT from the power dissipated. The method calculates ZT from the ratio P(xcfx890)/P(2xcfx890), where P(xcfx890) and P(2xcfx890) are, respectively, amplitudes of the power at the reference frequency and at two times the reference frequency. The method can use various techniques to measure the power dissipated, including monitoring infrared emission from each of the library members during application of the oscillatory voltage.
When the method includes identification of library members that are semiconductors and subsequent determination of ZT, the method usually includes isolating single-phase materials of the semiconducting library members. Isolation of single-phase materials may include determining the number of phases present in each of the semiconducting library members, and/or identifying compositions of the phases. The method can use various techniques to determine the number and/or composition of phases, including differential scanning calorimetry, x-ray diffraction, energy dispersive x-ray spectroscopy, secondary ion mass spectroscopy, x-ray fluorescence spectroscopy, and the like.
In addition, the present invention generally provides an apparatus for discovering semi-conducting or thermoelectric materials using combinatorial techniques. The apparatus includes a combinatorial library of materials comprised of thin films arrayed on a substrate, and a device for identifying semiconducting members of the combinatorial library of materials. The substrate can be transparent or opaque to the radiation of interest, and may comprise multiple layers. For example, an infrared transparent substrate may comprise an IR transparent membrane disposed on a support layer, with the membrane interposed between the thin films and the support layer. The support layer may have apertures aligned with the thin films that allow transmission of IR radiation through an otherwise IR opaque support layer. Similarly, an IR opaque substrate may comprise an infrared reflective layer disposed on a support layer, the infrared reflective layer interposed between the thin films and the support layer. Useful devices for identifying semiconductors include an FTIR spectrometer, which can be outfitted with a computer-controlled stage for positioning the thin films arrayed on the substrate.
Alternatively, or in addition to the device for identifying semiconductors, the apparatus may include a device for measuring ZT: a voltage source for applying an oscillatory electrical potential across members of a combinatorial library arrayed on a substrate, and a device for measuring power dissipated by each of the library members during application of the oscillatory electrical potential. The members of the combinatorial library are generally arrayed on a substrate having a lower or comparable thermal conductivity than the library members and each of the library members are typically connected, in parallel, to the voltage source. Useful devices for measuring power dissipated by each of the library members include infrared detectors, such as bolometers, photodiodes, focal plane arrays, thermcouples, thermistors, resistance thermometric devices, and the like. The apparatus may also include a computer for transforming power from the time domain to the frequency domain, and a vacuum chamber for containing the library during application of the oscillatory electrical potential.
When the apparatus includes devices for identification of semiconducting library members and for subsequent determination of ZT, the apparatus usually includes a device for isolating single-phase materials of library members that were identified as semiconductors. Devices for isolating single-phase materials include a differential scanning calorimeter, a scanning x-ray diffraction unit, an energy dispersive x-ray spectrometer, a secondary ion mass spectrometer, an x-ray fluorescence spectrometer, and the like.